Visualizing vascular structures

ABSTRACT

The present invention relates to visualizing vascular structures. In order to provide further improved digital subtraction angiography, a device (10) for visualizing vascular structures is provided that comprises a data provision unit (12), a data processing module (14), and an output unit (16). The data provision unit is configured to provide a first sequence of non-contrast X-ray images of a region of interest of a patient for use as raw X-ray mask images. The data provision unit is also configured to provide a second sequence of contrast X-ray images of the region of interest of a patient for use as raw X-ray live-images. The processing unit is configured to perform a first spatial subtraction for the first sequence of non-contrast X-ray images resulting in a first sequence of spatial-subtracted mask images. The processing unit is also configured to perform a second spatial subtraction for the second sequence of contrast X-ray images resulting in a second sequence of spatial-subtracted X-ray live-images. The processing unit is further configured to perform a temporal subtraction by subtracting the spatial-subtracted mask images from the spatial-subtracted X-ray live-images resulting in a sequence of spatial-temporal subtracted X-ray live-images. The output unit is configured to output the sequence of spatial-temporal subtracted X-ray live-images.

CROSS-REFERENCE TO PRIOR APPLICATIONS

This application is the U.S. National Phase application under 35 U.S.C.§ 371 of International Application No. PCT/EP2017/082895, filed on Dec.14, 2017, which claims the benefit of European Patent ApplicationNo.16306688.9, filed on Dec. 15, 2016. These applications are herebyincorporated by reference herein.

FIELD OF THE INVENTION

The present invention relates to visualizing vascular structures, andrelates in particular to a device for visualizing vascular structures,to a medical imaging system and to a method for visualizing vascularstructures.

BACKGROUND OF THE INVENTION

In interventional X-ray imaging, digital subtraction angiography (DSA)may be performed to visualize vascular structures. To emphasize thevascular structures, contrast agent is injected. Images may be obtainedby subtracting a non-injected mask frame from injected frames. In caseof motion, and in particular breathing or bowel gas motion, the qualityof subtracted images may be affected and the resulting artefacts mayinfluence the diagnostic value of the images. To reduce this effect, asan example, the motion vector field between mask and current frame isdetected. The mask is then warped in order to be better aligned with thecurrent frame before subtraction is performed.

For an improved result, WO2016110420 describes digital subtractionangiography with a double-temporal subtraction for artefacts reduction.Also, WO2016083068 describes digital subtraction angiography withseparate compensation for breathing and cardiac motion artefacts.

SUMMARY OF THE INVENTION

There may be a need to provide further improved digital subtractionangiography.

The object of the present invention is solved by the subject-matter ofthe independent claims; further embodiments are incorporated in thedependent claims. It should be noted that the following describedaspects of the invention apply also for the device for visualizingvascular structures, for the medical imaging system and for the methodfor visualizing vascular structures.

According to an aspect, a device for visualizing vascular structures isprovided. The device comprises a data provision unit, a data processingmodule, and an output unit. The data provision unit is configured toprovide a first sequence of non-contrast X-ray images of a region ofinterest of a patient for use as raw X-ray mask images. The dataprovision unit is also configured to provide a second sequence ofcontrast X-ray images of the region of interest of a patient for use asraw X-ray live-images. The processing unit is configured to perform afirst spatial subtraction for the first sequence of non-contrast X-rayimages resulting in a first sequence of spatial-subtracted mask images.The processing unit is also configured to perform a second spatialsubtraction for the second sequence of contrast X-ray images resultingin a second sequence of spatial-subtracted X-ray live-images. Theprocessing unit is further configured to perform a temporal subtractionby subtracting the spatial-subtracted mask images from thespatial-subtracted X-ray live-images resulting in a sequence ofspatial-temporal subtracted X-ray live-images. The output unit isconfigured to output the sequence of spatial-temporal subtracted X-raylive-images.

Since X-ray images are transparency images and observed motion mayresult from the super-imposition of various motion layers with differentcharacteristics, the spatial subtraction supports in compensating forsources of motion.

According to an example, a display is provided, which is configured todisplay the sequence of spatial-temporal subtracted X-ray live-images.

According to a further aspect, also a medical imaging system forvisualizing vascular structures is provided. The medical imaging systemcomprises an X-ray imaging device comprising an X-ray source and anX-ray detector, and a device for visualizing vascular structuresaccording to one of the examples above. The X-ray imaging device isconfigured to provide at least a plurality of X-ray images of a regionof interest of a patient as the second sequence of contrast X-rayimages.

According to a further aspect, also a method for visualizing vascularstructures is provided. The method comprises the following steps:

a1) providing a first sequence of non-contrast X-ray images of a regionof interest of a patient for use as raw X-ray mask images;

a2) performing a first spatial subtraction for the first sequence ofnon-contrast X-ray images resulting in a first sequence ofspatial-subtracted mask images;

b1) providing a second sequence of contrast X-ray images of the regionof interest of a patient for use as raw X-ray live-images;

b2) performing a second spatial subtraction for the second sequence ofcontrast X-ray images resulting in a second sequence ofspatial-subtracted X-ray live-images; and

c) performing a temporal subtraction by subtracting thespatial-subtracted mask images from the spatial-subtracted X-raylive-images resulting in a sequence of spatial-temporal subtracted X-raylive-images.

According to an example, it is provided a step d) of displaying thesequence of spatial-temporal subtracted X-ray live-images.

According to an example, the first spatial subtraction and/or the secondspatial subtraction comprises: estimating a soft tissue map ofsoft-tissue for each image of the sequence of X-ray images, andsubtracting the estimated soft tissue map from the X-ray image.

According to an example, the estimating of the soft-tissue comprisesharmonization techniques. Spatial low-frequencies are estimated in lowmulti-resolution pyramid levels, and un-wanted polarities or frequenciesare attenuated or dumped.

According to an embodiment, it is proposed to reduce motion artefacts inabdominal DSA through a spatio-temporal approach. This performs a strongreduction of the amplitude of soft tissues in the spatial domain priortemporal subtraction that results in a reduction of motion artefactsafter temporal subtraction. The proposed approach performs better thancurrent methods used to compensate for motion artefacts aftersubtraction. The present approach overcomes the difficulty of motionestimation in transparent layers by a combination of spatial andtemporal subtractions. This aims at correcting a combination of variousmotion sources and types.

It is proposed to prevent the occurrence of subtraction artefacts ratherthan correct for artefacts. In particular, two spatial subtractions areperformed, one in the mask frames and one in the live frames, prior to atemporal subtraction of mask and live frames. As a result, the amplitudeof motion artefacts is reduced.

In particular, use is made of a succession of two different types ofsubtractions, namely a spatial subtraction followed by a temporalsubtraction. For the mask frame, a map of soft-tissue (mainly slowlyvarying) is estimated. The soft tissue map is subtracted from the maskframe (spatial subtraction) and the spatially subtracted mask is storedas SS-mask. For each live frame of the sequence, a map of soft-tissue isestimated. The soft tissue map is subtracted from the current live frame(spatial subtraction). The spatially subtracted live frame is stored asSS-live. Then, the temporal subtraction between SS-mask and SS-live isperformed (temporal subtraction). A spatio-temporal subtracted STS-liveoutput is obtained. The DSA output can then be displayed.

The invention may be used in all DSA imaging involving breathing orbowel gas motion, such as abdominal imaging for example.

These and other aspects of the present invention will become apparentfrom and be elucidated with reference to the embodiments describedhereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the invention will be described in thefollowing with reference to the following drawings:

FIG. 1 shows a schematic setup of an example of a device for visualizingvascular structures.

FIG. 2 shows an example of medical imaging system.

FIG. 3 shows an example of a method for visualizing vascular structures.

FIG. 4 shows examples for images used for the method of FIG. 3.

FIG. 5 shows the examples of FIG. 4 as photographic images.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1 shows an example of a device 10 for visualizing vascularstructures. The device 10 comprises a data provision unit 12, a dataprocessing module 14, and an output unit 16.

The data provision unit 12 is configured to provide a first sequence ofnon-contrast X-ray images of a region of interest of a patient for useas raw X-ray mask images. The data provision unit 12 is also configuredto provide a second sequence of contrast X-ray images of the region ofinterest of a patient for use as raw X-ray live-images.

The processing unit 14 is configured to perform a first spatialsubtraction for the first sequence of non-contrast X-ray imagesresulting in a first sequence of spatial-subtracted mask images. Theprocessing unit 14 is also configured to perform a second spatialsubtraction for the second sequence of contrast X-ray images resultingin a second sequence of spatial-subtracted X-ray live-images. Theprocessing unit 14 is further configured to perform a temporalsubtraction by subtracting the spatial-subtracted mask images from thespatial-subtracted X-ray live-images resulting in a sequence ofspatial-temporal subtracted X-ray live-images.

The output unit 16 is configured to output the sequence ofspatial-temporal subtracted X-ray live-images.

The data provision unit 12 is also referred to as provision unit or asdata input interface. The processing module 14 is also referred to asdata processing module. The output unit 16 is also referred to as dataoutput unit or data output interface.

The term “unit” relates to a functional component or functional unit.The unit can be provided as an integrated part or module providing saidfunction. The unit can also be provided as a separated unit, part ormodule.

As an example, the data provision unit 12, the data processing module14, and the output unit 16 are provided integrated as indicated withframe 18.

The term “to output the sequence of spatial-temporal subtracted X-raylive-images” relates to providing the data of the sequence of thespatial-temporal subtracted X-ray live-images for further purposes. Asan example, the data is used for displaying the respective sequence. Asanother example, the data is for further data processing steps such ascomparison with pre-captured images and further live images, or acombination with pre-captured images and further live images.

As an option, indicated with hashed lines, a display 22 is provided,which is configured to display the sequence of spatial-temporalsubtracted X-ray live-images.

The display receives the image data from the output unit. In an example,the display is provided as a main or subsidiary monitor in a medicalimaging system.

FIG. 2 shows an example of a medical imaging system 30 for visualizingvascular structures. The medical imaging system 30 comprises an X-rayimaging device 32 and a device 34 for visualizing vascular structuresaccording to one of the above examples.

X-ray imaging device 32 comprises an X-ray source 36 and an X-raydetector 38 to image an object 40, for example a patient, arranged on apatient support 42. The X-ray imaging device 32 is shown as a C-arcsystem, but also other X-ray imaging devices are provided. The X-rayimaging device 32 is configured to provide at least a plurality of X-rayimages of a region of interest of a patient as the second sequence ofcontrast X-ray images.

The X-ray imaging device provides the live X-ray image data of apatient.

In an example, the X-ray imaging device is also configured to provide afirst sequence of non-contrast X-ray images of a region of interest of apatient.

FIG. 3 shows an example of a method 100 for visualizing vascularstructures, comprising the following steps:

-   -   In a first step 102, also referred to as step a1), a first        sequence of non-contrast X-ray images of a region of interest of        a patient is provided for use as raw X-ray mask images.    -   In a second step 104, also referred to as step a2), a first        spatial subtraction is performed for the first sequence of        non-contrast X-ray images resulting in a first sequence of        spatial-subtracted mask images 106.    -   In a third step 108, also referred to as step b1), a second        sequence of contrast X-ray images of the region of interest of a        patient is provided for use as raw X-ray live-images.    -   In a fourth step 110, also referred to as step b2), a second        spatial subtraction is performed for the second sequence of        contrast X-ray images resulting in a second sequence of        spatial-subtracted X-ray live-images 112.    -   In a fifth step 114, also referred to as step c), a temporal        subtraction is performed by subtracting the spatial-subtracted        mask images from the spatial-subtracted X-ray live-images        resulting in a sequence 116 of spatial-temporal subtracted X-ray        live-images.

The term “non-contrast X-ray images” relates to X-ray images of theregion of interest without the application of any contrast enhancingsubstance, i.e. without injected or otherwise supplied contrast agent.

The term “contrast X-ray images” relates to X-ray images of the regionof interest with applied contrast enhancing substances, e.g. withinjected or otherwise supplied contrast agent. The contrast agent may beprovided into the bloodstream or blood flow in the vessels and thusenhances the visibility of the vasculature structure.

In an example, the first spatial subtraction is performed for each imageof the first sequence of non-contrast X-ray images.

In an example, the second spatial subtraction is performed for eachimage of the second sequence of contrast X-ray images.

The raw mask images are pre-processed e.g. by subtracting soft-tissue,before the temporal subtraction takes place.

The same approach is also provided for the live images. In other words,the live images are also pre-processed before the temporal subtractiontakes place.

Both pre-processing sub-steps must be done before the temporalsubtraction.

The “raw X-ray mask images” can also be referred to as raw mask images,or basic X-ray mask data or basic mask data or X-ray mask basics or maskbasics.

The “raw X-ray live-images” can also be referred to as raw live-images,or basic X-ray live-data or live-data or X-ray live-basics.

The “spatial-temporal subtracted X-ray live-images” can also be referredto as vessel-enhanced X-ray live-images or enhanced X-ray live-data orenhanced live-images or enhanced live-data.

The images of the first and second sequence are also referred to asframes, e.g. first frames and second frames.

By providing two spatial subtractions in both the mask and the liveframes, prior to temporal subtraction, i.e. before the mask image issubtracted from the live image, the occurrence of subtraction artefactsis prevented or at least reduced. The need for correcting for artefactsafter the subtraction is thus reduced. The two spatial subtractions inboth the mask and the live frames reduce the amplitude of motionartefacts in the subtraction result.

The subtractions in steps a2) and b2) are referred to as spatialsubtractions, because here content within the spatial domain of theimage is used for the subtraction.

The subtraction in step c) is referred to as a temporal subtraction,because here images of different sequences acquired at different pointsin time, i.e. images of different points on the time domain, are usedfor the subtraction. The live image is used and by subtracting theprevious image, the difference remains, i.e. those parts of the imagewhere contrast agent is present in the image. Hence, a bettervisualization of the vasculature is provided. Briefly said, thesubtraction reduces the resulting image to the differing parts. Tissuerelated content that is not subject to contrast injection is eliminated.

However, motion between the two images (i.e. the two sequences) mayresult in artefacts. Part of this can be compensated by identifyingdifferent states of a motion cycle and by assigning images of the firstsequence to respective matching images of matching motion states of thesecond sequence.

In an example, the steps a1) and a2) are performed prior to acquiringthe second sequence of images.

In another example, the steps a1) and a2) are performed parallel to thesteps b1) and b2).

The images that form the basis for step a1) are nevertheless acquired ata different, i.e. earlier, point in time than the acquisition of thesecond sequence of images.

In an example, a spatio-temporal image subtraction for abdominal DSA isprovided.

As an example, in interventional X-ray, digital subtract angiography(DSA) may be performed to visualize vascular structures. The images areobtained by subtracting a non-injected mask frame from injected frames.In case of motion, and in particular breathing or bowel gas motion, thequality of subtracted images may be strongly reduced and the resultingartefacts frequently reduce the diagnostic value of the images.

X-ray images are transparency images and observed motion may result fromthe super-imposition of various motion layers with differentcharacteristics. Further accurate compensation is provided for all thesources of motion with the present approach.

In an example, the spatio-temporal approach is combined with a selectionof an optimal frame in the mask sub-sequence to further improve STS-liveimages.

In an example, it is provided to apply rigid motion compensation betweenmask and live frames.

In an example, the current temporal subtraction is swapped by amotion-compensated temporal subtraction.

In an example, motion artefacts are reduced in abdominal DSA through aspatio-temporal approach. The method performs a strong reduction of theamplitude of soft tissues in the spatial domain prior temporalsubtraction that results in a reduction of motion artefacts aftertemporal subtraction. The proposed approach shows an improvedperformance than current methods used to compensate for motion artefactsafter subtraction.

In an example, temporal subtraction is a motion compensated subtraction,which is a subtraction of the spatial-subtracted motion compensated maskimages from the spatial-subtracted X-ray live-images.

As an option, indicated in FIG. 3 with hashed lines, it is provided afurther step 118, also referred to as step d), in which the sequence ofspatial-temporal subtracted X-ray live-images are displayed.

In an example, not further shown, the first spatial subtraction and/orthe second spatial subtraction comprises: estimating a soft tissue mapof soft-tissue for each image of the sequence of X-ray images, andsubtracting the estimated soft tissue map from the X-ray image.

The subtraction of the soft-tissue reduces the range of amplitudeswithin the image.

In an example, for the mask frame, a map of soft-tissue (mainlyspatially slowly varying) is estimated. Further, the soft tissue map issubtracted from the mask frame (which is referred to as the spatialsubtraction). The spatially subtracted image(s) is (are) stored as aspatially subtracted mask.

In an example, for each live frame, a map of soft-tissue is estimated.The soft tissue map is subtracted from the current live frame (which isreferred to as the spatial subtraction). The spatially subtractedframe(s) is (are) stored as a spatially subtracted live images.

In an example, the temporal subtraction is performed between the spatialsubtraction mask and the spatial subtraction live data (which isreferred to as the temporal subtraction).

In an example, a spatio-temporal (or spatial-temporal) subtracted liveoutput is obtained as a result, or output, of a digital subtractionangiography (DSA).

In an option, the output of the digital subtraction angiography isdisplayed.

The estimation of a soft-tissue map can be done by so-calledharmonization techniques. Spatial low-frequencies can be estimated inlow multi-resolution pyramid levels, and the resulting estimationfurther processed to damp un-wanted polarities or frequencies (vesselsshow a known contrast polarity, and their maximum size is known).

In an example, not further shown, the estimating of the soft-tissuecomprises harmonization techniques. Spatial low-frequencies areestimated in low multi-resolution pyramid levels, and un-wantedpolarities or frequencies are attenuated or dumped. For example, vesselsshow a known contrast polarity, and their maximum size is known. Thisallows to flatten the image.

FIG. 4 shows a further example of the image processing.

In the left column, a mask image 112 is provided showing a region ofinterest of a patient, e.g. a vertebra structure 123. After a spatialsubtraction step 124, a spatial subtracted mask image 126 is provided.

In the right column, a live input image 128 is provided showing theregion of interest of a patient, e.g. a vertebra structure, but alsowith contrast injected vessels 125. After a spatial subtraction step130, a spatial subtracted live image 132 is provided.

Following, a temporal subtraction 134 is provided resulting in an outputlive image 136 showing the vasculature structure in an improved way.

FIG. 4 shows the images with lines only. FIG. 5 shows the images of FIG.4 in a photographic manner.

In another exemplary embodiment of the present invention, a computerprogram or a computer program element is provided that is characterizedby being adapted to execute the method steps of the method according toone of the preceding embodiments, on an appropriate system.

The computer program element might therefore be stored on a computerunit, which might also be part of an embodiment of the presentinvention. This computing unit may be adapted to perform or induce aperforming of the steps of the method described above. Moreover, it maybe adapted to operate the components of the above described apparatus.The computing unit can be adapted to operate automatically and/or toexecute the orders of a user. A computer program may be loaded into aworking memory of a data processor. The data processor may thus beequipped to carry out the method of the invention.

This exemplary embodiment of the invention covers both, a computerprogram that right from the beginning uses the invention and a computerprogram that by means of an up-date turns an existing program into aprogram that uses the invention.

Further on, the computer program element might be able to provide allnecessary steps to fulfil the procedure of an exemplary embodiment ofthe method as described above.

According to a further exemplary embodiment of the present invention, acomputer readable medium, such as a CD-ROM, is presented wherein thecomputer readable medium has a computer program element stored on itwhich computer program element is described by the preceding section. Acomputer program may be stored and/or distributed on a suitable medium,such as an optical storage medium or a solid-state medium suppliedtogether with or as part of other hardware, but may also be distributedin other forms, such as via the internet or other wired or wirelesstelecommunication systems.

However, the computer program may also be presented over a network likethe World Wide Web and can be downloaded into the working memory of adata processor from such a network. According to a further exemplaryembodiment of the present invention, a medium for making a computerprogram element available for downloading is provided, which computerprogram element is arranged to perform a method according to one of thepreviously described embodiments of the invention.

It has to be noted that embodiments of the invention are described withreference to different subject matters. In particular, some embodimentsare described with reference to method type claims whereas otherembodiments are described with reference to the device type claims.However, a person skilled in the art will gather from the above and thefollowing description that, unless otherwise notified, in addition toany combination of features belonging to one type of subject matter alsoany combination between features relating to different subject mattersis considered to be disclosed with this application. However, allfeatures can be combined providing synergetic effects that are more thanthe simple summation of the features.

While the invention has been illustrated and described in detail in thedrawings and foregoing description, such illustration and descriptionare to be considered illustrative or exemplary and not restrictive. Theinvention is not limited to the disclosed embodiments. Other variationsto the disclosed embodiments can be understood and effected by thoseskilled in the art in practicing a claimed invention, from a study ofthe drawings, the disclosure, and the dependent claims.

In the claims, the word “comprising” does not exclude other elements orsteps, and the indefinite article “a” or “an” does not exclude aplurality. A single processor or other unit may fulfil the functions ofseveral items re-cited in the claims. The mere fact that certainmeasures are re-cited in mutually different dependent claims does notindicate that a combination of these measures cannot be used toadvantage. Any reference signs in the claims should not be construed aslimiting the scope.

The invention claimed is:
 1. A device for visualizing vascularstructures, comprising: a data provision unit; a data processing module;and an output unit; wherein the data provision unit is configured toprovide a first sequence of non-contrast X-ray images of a region ofinterest of a patient for use as raw X-ray mask images; and to provide asecond sequence of contrast X-ray images of the region of interest of apatient for use as raw X-ray live-images; wherein the data processingmodule is configured to perform a first spatial subtraction for thefirst sequence of non-contrast X-ray images resulting in a firstsequence of spatial-subtracted mask images; and to perform a secondspatial subtraction for the second sequence of contrast X-ray imagesresulting in a second sequence of spatial-subtracted X-ray live-images;and to perform a temporal subtraction by subtracting thespatial-subtracted mask images from the spatial-subtracted X-raylive-images resulting in a sequence of spatial-temporal subtracted X-raylive-images; wherein the output unit is configured to output thesequence of spatial-temporal subtracted X-ray live-images.
 2. The deviceaccording to claim 1, wherein a display is provided, which is configuredto display the sequence of spatial-temporal subtracted X-raylive-images.
 3. A medical imaging system for visualizing vascularstructures, comprising: an X-ray imaging device comprising an X-raysource and an X-ray detector; and the device for visualizing vascularstructures according to claim 1; wherein the X-ray imaging device isconfigured to provide at least a plurality of X-ray images of a regionof interest of a patient as the second sequence of contrast X-rayimages.
 4. A method for visualizing vascular structures, comprising thefollowing steps: a1) providing a first sequence of non-contrast X-rayimages of a region of interest of a patient for use as raw X-ray maskimages; a2) performing a first spatial subtraction for the firstsequence of non-contrast X-ray images resulting in a first sequence ofspatial-subtracted mask images; b1) providing a second sequence ofcontrast X-ray images of the region of interest of a patient for use asraw X-ray live-images; b2) performing a second spatial subtraction forthe second sequence of contrast X-ray images resulting in a secondsequence of spatial-subtracted X-ray live-images; and c) performing atemporal subtraction by subtracting the spatial-subtracted mask imagesfrom the spatial-subtracted X-ray live-images resulting in a sequence ofspatial-temporal subtracted X-ray live-images.
 5. The method accordingto claim 4, wherein temporal subtraction is a motion compensatedsubtraction, which is a subtraction of the spatial-subtracted motioncompensated mask images from the spatial-subtracted X-ray live-images.6. The method according to claim 4, wherein it is provided: d)displaying the sequence of spatial-temporal subtracted X-raylive-images.
 7. The method according to claim 4, wherein the firstspatial subtraction and/or the second spatial subtraction comprises:estimating a soft tissue map of soft-tissue for each image of thesequences of X-ray images; and subtracting the estimated soft tissue mapfrom the X-ray image.
 8. The method according to claim 7, wherein theestimating of the soft-tissue comprises harmonization techniques;wherein spatial low-frequencies are estimated in low multi-resolutionpyramid levels, and un-wanted polarities or frequencies are attenuatedor dumped.
 9. A computing unit excuting a computer program element forcontrolling a device for visualizing vascular structures, comprising: adata provision unit; a data processing module; and an output unit;wherein the data provision unit is configured to provide a firstsequence of non-contrast X-ray images of a region of interest of apatient for use as raw X-ray mask images; and to provide a secondsequence of contrast X-ray images of the region of interest of a patientfor use as raw X-ray live-images; wherein the data processing module isconfigured to perform a first spatial subtraction for the first sequenceof non-contrast X-ray images resulting in a first sequence ofspatial-subtracted mask images; and to perform a second spatialsubtraction for the second sequence of contrast X-ray images resultingin a second sequence of spatial-subtracted X-ray live-images; and toperform a temporal subtraction by subtracting the spatial-subtractedmask images from the spatial-subtracted X-ray live-images resulting in asequence of spatial-temporal subtracted X-ray live-images; wherein theoutput unit is configured to output the sequence of spatial-temporalsubtracted X-ray live-images, which, when being executed by thecomputing unit, is adapted to perform the method steps of claim
 4. 10. Anon-transitory computer readable medium having stored thereon thecomputer program element of claim 9.